SAN FRANCISCO — Law enforcement experts call residential entryways the “fatal funnel” —and that is where two Oakland police officers met death when they confronted a barricaded gunman, who had already killed two other officers hours earlier.

The SWAT team’s decision to rush an apartment rather than wait out an armed man is expected to be the subject of Special Weapons and Tactics training classes across the country, experts said.

And the Oakland Police Department said Friday that its internal investigation of the March 21 killings will include a review by outside SWAT experts.

It was the deadliest incident involving California officers since 1970.

Lawyers, police trainers and others expect the examination to determine whether police followed departmental guidelines and whether they made tactical or procedural mistakes.

Police say parolee Lovelle Mixon shot motorcycle patrolmen Mark Dunakin and John Hege with a handgun during a traffic stop, then later killed SWAT team members Ervin Romans and Daniel Sakai with an assault weapon. Another SWAT officer was injured, and Mixon was killed.

Police have not said why Dunakin and Hege stopped Mixon. The two patrolmen were killed without drawing their own weapons.

After that shooting, investigators said, Mixon fled a few blocks away to a sister’s apartment. An anonymous tip exposed Mixon’s hiding place— and about two hours after the traffic stop, the SWAT team stormed through the apartment’s front door, detonating flash-bang devices.

Much of the focus outside the Oakland department has been on the SWAT team, because the highly trained officers were killed while in pursuit of someone they believed already had gunned down two of their colleagues.

The department’s written guidelines for resolving such situations begins with “containment” but also includes “breach and entry of objective site” among 10 options.

“In any event, a resolution will be accomplished with the utmost consideration given to the safety of citizens, police personnel and all involved parties,” states the department’s general orders for SWAT operations.

Police officials have said the SWAT team entered the apartment to clear and search it, but precisely what prompted their decision is unclear.

The department turned down requests to interview a SWAT team commander and Police Chief Howard Jordan.

“The whole incident is still under investigation and OPD is not releasing any details or reports,” police spokesman Jeff Thomason said in an April 2 e-mail. “This is to protect the integrity of the investigation until it can be completed.”

Law enforcement experts said that without knowing more about the circumstances, it is difficult to second-guess the officers’ actions.

Some speculated that the SWAT team decided to enter the apartment because they feared waiting for the gunman to give up could result in more violence and harm to building occupants.

They said the officers faced one of the most dangerous scenarios in law enforcement: a gunman holed up in a residence and presumably training his sights on the entryway.

“I don’t see where it was necessarily prudent for the officers to go in when they went in,” said Michael Lyman, a criminal justice professor at Columbia College in Missouri, who based his comments on publicly available information. “Unless there is compelling need to go through that fatal funnel, don’t do it.”

Ed Nowicki, executive director of the International Law Enforcement Educators and Trainers Association based in Twin Lakes, Wis., said the people inside the building always have the advantage over those outside, simply because they have more knowledge.

“It’s their turf, not your turf,” he said.

Nowicki said SWAT officers should avoid storming a residence unless there are strong signals that a gunman is on the verge of more violence.

“You go in pretty much as a last resort,” Nowicki said. “And if you don’t have to, you don’t. If you can wait them out, you wait them out.”

Experts widely agree that the Oakland police shootings will be one of the most widely studied cases in law enforcement training.

It will certainly alter SWAT training in some way, said Robert Stresak, a spokesman for the California Commission on Peace Officer Standards and Training.

“It is a watershed law enforcement moment,” Stresak said. “Obviously, when four heroes are killed, something didn’t go right.”

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